The invention relates to a system for transmitting electrical signals via the body of a living creature and comprising a transmitter on one side of the body that can output a signal that can be fed applied to the body, passed through the body, and outputted from the body. The system also has a receiver on the other side of the body that can receive the signal transmitted by the transmitter and passed through the body.
Such systems are known from the prior art. A common feature of such systems is that electrical signals are fed into the human body, and are then distributed over the entire body because of the conductive skin surface. These systems are used in the field of presence detection, seat occupancy, detection of the proximity to a switching element, gesture detection and in the differentiation between drivers and passengers in the actuation of switching or operating elements.
DE 43 01 160 describes such a system in which a vehicle's equipment and devices are provided with operating elements as control elements. In this connection electrical signal-output and input elements are provided in the driver's seat and in the front passenger seat and in an operating element. When contact is made with the operating element a signal path is formed through which a signal can be supplied to an evaluation circuit. When detected, this signal allows the evaluation circuit to detect whether the operating element is actuated by the driver or the passenger. To this end signals are fed to the driver and into the passenger at different frequencies or at different times, so that the evaluation circuit of the receiver can distinguish between actuation by the driver and actuation by the passenger based on the respective signal. Accordingly the evaluation circuit is implemented as a frequency and/or time selection circuit.
DE 10 2007 056 802 describes such a system that allows a functional component of the vehicle to be controlled from at least two different seats by at least one operating element. In this case electrical signals are coupled capacitively into the body of the person located in each of the seats. In the receiver of this apparatus, differences in the signals allow a distinction to be made between actuation of the operating element by the driver located in one seat or the passenger located in the other seat.
A method is known from DE 10 2008 047 803 for transmitting signals with at least two transmitters operating with near-field technology. The respective transmitters output their signals or data in respective time windows.
U.S. Pat. No. 8,665,077 describes a circuit for selective switching signal generation. The circuit arrangement is used in a method for selective generation of a switching signal specifically for controlling functional components of a motor vehicle. In this case respective specific electrical signals are each fed to the body via actuating or operating elements and applied to the driver or passenger therein via a coupling surface or antenna provided in the vehicle seat.
The systems known from the prior art are not entirely suitable for use in motor vehicles, since they frequently do not meet the requirements for the availability of the signal. In particular the systems that operate sequentially or by time multiplexing are not suitable for permanent and precise detection of fast approaches, changes of position or gestures, since the necessary signal is not continuously available. Moreover such systems must in particular meet requirements for availability taking into account the resistance to interference. A system that regularly functions not at all or only unsatisfactorily because of interference signals frequently emitted during operation of the motor vehicle in the range of the signal frequencies used, leads to a high degree of dissatisfaction in the users of such systems. This applies in particular with regard to the evaluation and the plausibility checking of signals with which decisions relevant to safety are associated, for example the anti-pinch control in an electric sliding roof. Accordingly the systems described above are not suitable for meeting all the requirements occurring in operation of the motor vehicle.
Furthermore systems in which the so-called frequency hopping method is actively applied are known from the prior art systems. Such systems operate in comparatively narrow frequency bands. Therefore the probability exists that the working frequency of the corresponding system is distorted by an interference signal at the same frequency, so that the entire system cannot meet the requirements placed on it. If the system recognizes a distorted signal, then it is switched to a different frequency according to a pre-programmed scheme. This operation is repeated until an interference-free frequency has been found.
This procedure can be implemented comparatively simply in such systems in which a stable data link exists between the transmitter and the receiver, since then in the event of interference the receiver sends a corresponding signal or corresponding information to the transmitter to change the frequency.
The transmitter alone is not usually capable of recognizing an interference signal. For this purpose the transmitter itself would have to have a receiver. The expenditure on technology and design for this is generally too high. Moreover the transmitter could not recognize interference signals if the interference signal only occurs near the receiver.
Furthermore such systems in which no data link exists between the receiver and the transmitter are widespread. This applies for example in the case of transmitters carried on the body or locators carried on the body. However, also in motor vehicle applications locators are built into the vehicle seats, and the receivers are for example in the operating part of an air conditioning system and/or in the roof module for controlling a sliding roof. In such applications a data link between the receivers and the transmitters would be technically complex and associated with considerable costs.